Small-sized motor-actuated lifting apparatus generally used have shortcomings because of their own structure. First, the lift rod is threadedly engaged with a vertical screw or bolt in a simple way so that the operator has to pay close attention to the height the lift rod has gone to, lest the lift rod go beyond the top of the screw and unthreads itself. Then, when lowering the lift rod, the operator has to watch it carefully for fear that the lift rod goes down against a bottom stop and damages the motor. Such kind of structure is obviously faulty because it causes a lot of inconvenience to the operator as well as possible damage to the motor.
Something has been done by those who tried to improve the lifting apparatus. They designed a bolt with screw thread in the middle only, both ends of the bolt having smooth shanks. In this way, as they hoped, the lift rod will not go too far up or down. That is, the lift rod no longer separates itself from the bolt, nor does it press against a bottom so tightly as to cause damage to the motor. However, the bolt of this kind has its own faults too. For example, when the lift rod is raised to the top or lowered to the bottom and is to be reversed, an external force is needed to push or pull it back to the place where its internal thread can threadedly re-engage the thread of the bolt. It is inconvenient for the operator to handle this.
Moreover, when the two types of lifting apparatus--the traditional one and the partly modified one as mentioned above are repeatedly used to jack things up for the same distance, the operator has always to keep an eye on the distance from low to high so as to cut the electricity to the motor at the right time. Of course it is very inconvenient to do so.